This invention relates generally to signal processing systems and more particularly to beamforming controls for phased array antenna systems.
Phased array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front, or the cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The differences in phase or timing among the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antennas (or subarrays of antennas), has been accomplished by electronically shifting the phase of actuating signals or by introducing a time delay in actuating signals for selected antenna elements to sequentially excite the antenna elements to generate the desired direction of beam transmission from the antenna. Opto-electronic processing of beamforming signals for phased array antennas provides superior performance over conventional electronic-only processing by reason of, for example, increased bandwidth, compactness, and high performance signal control. Examples of such opto-electronic systems are disclosed in U.S. Pat. No. 5,117,239 of N. Riza entitled "Reversible Time Delay Beamforming Optical Architecture for Phased-Array Antennas", issued May 26, 1992, and in the following copending applications: Ser. No. 07/826,501, filed Jan. 27, 1992 and entitled "Time-Multiplexed Phased-Array Antenna Beam Switching System"; U.S. Pat. No. 5,191,339 filed Mar. 5, 1992 and entitled "Phased-Array Antenna Controller"; U.S. Pat. No. 5,187,487, filed Mar. 5, 1992 and entitled "A Compact Wide Tunable Bandwidth Phased Array Antenna Controller"; Ser. No. 07,900,877, filed Jun. 18, 1992 and entitled "Optical Time Delay Units for Phased Array Antennas"; and Ser. No. 07/955,165, filed Oct. 1, 1992 and entitled "Optical Controller With Independent Two-Dimensional Scanning", all of which are assigned to the assignee of the present invention and are incorporated herein by reference.
Ideally, a phased array antenna control system should have the ability to generate electromagnetic signals in the microwave to millimeter wavelength range, and should be light, compact, relatively immune to undesirable electromagnetic radiation, and straightforward to fabricate, operate, and maintain. Such a system also desirably has a wide antenna tunable bandwidth, and inertialess, motion-free high resolution beam scanning ability with application-dependent slow-to-fast scanning speeds.
It is additionally advantageous to have an analog beamforming control system that enables a large number of phase shift combinations to be generated and which thus provides enhanced transmit and receive beamforming capabilities. Such an analog system is in contrast to digital phase control from electronic phase shifters; the digital nature of such phase shifters limits the device to a fixed number of possible phase actuation signals. This limited number of possible actuation signals in turn limits the phase resolution achievable with the microwave devices, thus limiting the angular resolution of the scanned antenna beam. Further, in conventional electronically controlled phased array antennas, the digital microwave phase shifters are also typically used for correcting phase errors that result due to other microwave devices in the system. Because of the digital nature of the phase shifters, these phase errors can only be partially cancelled.
An optical control system can also use heterodyne detection between respective phase-shifted light beams in an optical signal pair to generate a scanning interference phase pattern. One example of such a system, using a liquid crystal pixel array for generating phase delays in one light beam of an optical signal pair, is disclosed in copending, commonly assigned, application of N. A. Riza entitled Phased-Array Antenna Controller, Ser. No. 07/847,155, cited above. Another method of generating a scanning pattern based on interference between light beam pairs is described in the article authored by N. Riza entitled "An Acoustooptic Phased Array Antenna Beamformer with Independent Phase and Carrier Control Using Single Sideband Signals", appearing in IEEE Photonics Technology Letters, Vol. 4, No. 2, February 1992.
Modulation of the optical signal can also be used to generate the desired electromagnetic transmission frequency, such as by direct modulation of the light source, typically a laser, at the desired frequency. At the high frequencies desired for optimum radar performance, however, such direct modulation techniques on the laser are seriously limited due to transients and harmonics in the laser output energy. Generation of optical interference patterns with two distinct laser beams, with subsequent heterodyne detection of the temporally-varying interference pattern, can be used to generate the desired carrier frequency of the electromagnetic signal transmitted from the antenna, as described in the article by G. Simmons and K. Purchase, "Optical Generation, Distribution, and Control of Microwaves Using Laser Heterodyne", IEEE Transactions on Microwave Theory and Techniques, Vol. 38, No. 5, May 1990, pp 667-669. This article notes that phase shifting of a microwave modulation superposed on an optical carrier is not straightforward to implement in a practical fashion, especially when thousands of channels may be involved, each requiring five or more bits of phase shift control. Simmons and Purchase suggest use of a semiconductor superlattice waveguide disposed in the path of one of the lasers or the use of deformable mirror devices, but neither of these devices have been readily fabricated in large scale high performance arrays suitable for radar applications. In particular, sub-micron motion accuracy is required for the deformable mirror devices to implement 0-2.pi. phase shift control at the optical wavelength. Such high performance motion accuracy is extremely difficult to achieve, and devices or arrays of this nature have so far not been built. Further, the deformable mirror devices do not provide a substantially inertialess optical phase modulation system.
It is accordingly an object of this invention to provide an opto-electronic signal processor that can generate carrier independent high quality (e.g., 6-8 bit) analog phase-based phased array antenna beam control.
It is a further object of this invention to provide a phase-based antenna controller that is relatively compact, lightweight and has an inertialess beam scanning structure and that can readily be connected to remote antenna sites via fiber optics.
Another object of this invention is to provide a phase-based antenna controller that has a wide (i.e., in a range from DC to several hundred GHz) tunable antenna bandwidth with stable phase-control and an independent, analog, phase-error calibration capability for all the elements in the array.
A further object of the present invention is to provide an optical beam processing technique that has low optical losses, low inter-channel crosstalk, and that is readily fabricated for use with a relatively large (e.g., &gt;1000) number of phased array antenna elements.
A still further object of this invention is to provide a optical controller for a phased array antenna having the ability to rapidly switch between a desired carrier frequency for transmission and a desired intermediate frequency for processing return radar signals.